This invention relates in general to active vehicle suspensions and in particular to a multi-channel hydraulic control unit for such a suspension.
Vehicle suspension systems control chassis motion during operation of the vehicle in order to isolate the vehicle load from irregularities in the terrain over which the vehicle travels. One such chassis motion, that is controlled by know suspension systems, is chassis roll. A vehicle experiences chassis roll during a turning maneuver. During chassis roll, the chassis tilts or “rolls” about the vehicle's fore-to-aft axis toward an outside direction of the turn.
In the past, vehicles have been provided with passive suspension systems that normally include a spring and damper connected in parallel between sprung and unsprung portions of the vehicle. Accordingly, a spring and damper is typically provided for each vehicle wheel. Passive suspension systems are generally self-contained and only react to loads applied to them.
More recently, active suspension systems have been developed that apply positive reactions to applied loads. Active suspension systems typically include hydraulic or pneumatic actuators that are coupled to the passive suspension system components. A typical prior art active suspension system with a hydraulically actuated active roll control system 10 is illustrated in FIG. 1. The roll control system 10 includes an Electronic Control Unit (ECU) 12 that is in electrical communication with at least one wheel speed sensor 14, a lateral accelerometer 16 and a steering angle detector 18 that together provide a means for sensing forces that cause the vehicle to roll.
The active roll control system 10 also includes a front anti-roll, or stabilizer, bar 20 and a front hydraulic actuator 22 associated with a pair of vehicle front wheels 24. Similarly, a rear anti-roll, or stabilizer, bar 26 and a rear hydraulic actuator 28 are associated with a pair of rear vehicle wheels 30. As illustrated in FIG. 1, both the front and rear hydraulic actuators comprise a piston and cylinder assembly. The front anti-roll bar 20 is mounted upon the vehicle body (not shown) with a first end connected via the front hydraulic actuator 22 and a front actuator strut 32 to one of a pair of front suspension arms 34. A second end of the front anti-roll bar 20 is connected via a front suspension strut 36 to the other of the front suspension arms 34. Similarly, the rear anti-roll bar 26 also is mounted upon the vehicle body (not shown) with a first end connected via hydraulic actuator 28 and a rear actuator strut 38 to one of a pair of rear suspension arms 40. A second end of the rear anti-roll bar 26 is connected via a rear suspension strut 42 to the other of the rear suspension arms 40.
The roll control system 10 further includes a pump 44 having an intake port connected by a first hydraulic fluid line 46 to a hydraulic fluid reservoir 48 and a discharge port connected by a second hydraulic fluid line 50 to a suspension fluid control device 52. A hydraulic fluid supply line 54 connects the suspension fluid control device 52 to a power steering valve assembly 56 while a hydraulic fluid discharge line 57 connects the power steering valve assembly 56 to the hydraulic fluid reservoir 48. The suspension fluid control device 52 is connected by front hydraulic supply lines 58 and 60 to the front and rear hydraulic actuators 22 and 28, respectively. The front and rear actuators 22 and 28 are also connected to the suspension control device 52 by hydraulic return lines 62 and 64, respectively, while the control device 52 is connected to the fluid reservoir 48 by a discharge line 66.
A fluid schematic drawing for the roll control system 10 is shown in FIG. 2 where components that are similar to components shown in FIG. 1 have the same numerical designators. Because so many vehicles are equipped with power steering, the power steering pump 44 is typically used to supply pressurized hydraulic fluid to both the power steering valve assembly 56 and the roll control system 10, as shown in FIG. 2. The roll control system 10 includes a three-position four-way directional control valve 72 for controlling the flow of hydraulic fluid to the front and rear cylinder and piston assemblies 22 and 26. A slidable spindle within the control valve is 72 moved axially by first and second pilot valves 72a and 72b, respectively, that are located in the ends of the directional valve. The first pilot valve 72a of the directional control valve 72 is connected to an normally closed digital solenoid valve 73a and a normally open digital solenoid valve 74a. Similarly, the second pilot valve 72b also is connected to an normally closed digital solenoid valve 73b and a normally open digital solenoid valve 74b. The directional control valve 72 is connected by a drain line 66 to the fluid reservoir 48 and by a feed line 75 to a priority valve 76. The priority valve 76 receives pressurized hydraulic fluid from the power steering pump 44 and divides the pump fluid flow between the power steering valve assembly 54 and the directional control valve 72. Typically, the power steering pump 44 is oversized by approximately 50 percent to provide flow for both the power steering and the roll control system 10. The pressure within the roll control system 10 is controlled by a two stage proportional pressure relief valve 77 having a main stage 78a controlled by a pilot stage 78b. As shown in FIG. 2, the pressure relief valve 77 is connected between the feed line 75 supplying pressurized hydraulic fluid to the directional control valve 72 and the reservoir 48. When there is no flow demand from the actuators 22 and 26, all of the fluid flow in line 75 for the roll control system 10 is diverted to the reservoir 48. Otherwise, the pressure of the hydraulic fluid supplied to the directional control valve 72 is controlled by the pressure relief valve 77 reducing the diverted flow to the reservoir 48. The roll control system 10 also includes a pair of pressure sensors 79 that monitor the hydraulic fluid pressure being supplied to the front and rear cylinder and piston assemblies 22 and 26 at the outlet ports A and B.
The roll control system 10 shown in FIG. 2 allows small valves to control large flows. Flow is supplied by a hydraulic pump which is powered in some fashion by the motor vehicle and directed to the actuators 22 and 26 by a three-position-four-way valve 72 that is controlled by two pairs of small ABS style solenoid valves 73 and 74. This allows for a minimum of power to be supplied by the vehicle to operate the system 10. Also, this valve arrangement allows a desirable failure mode where hydraulic fluid is locked into the hydraulic actuators 22 and 26, there by locking in the anti-roll bar in case of a system failure. If electrical power is not or can not be supplied to the small digital valves 73 and 74, the three-position-four-way valve 72 will maintain a center position locking fluid into the actuators 22 and 26 and causing the anti-roll bar to act as in a conventional suspension system. Fluid pressure is determined by a valve that supplies pressure proportional to an applied current. The use of a piloted operated pressure control valve 77 allows a small valve 78a to control large flows without large pressure drops added to the system.
During operation of the vehicle, the ECU 12 receives input signals from the wheel speed sensor 14, the lateral accelerometer 16 and the steering angle detector 18. The ECU 12 processes the input signals to determine any roll of the vehicle relative to the front and rear wheels 24 and 30. Based upon the roll determination, the ECU 12 activates the fluid control device 52 to supply pressurized hydraulic fluid to one end of the hydraulic actuators 22 and 28. In response, both of the pistons move axially within the cylinders to input a torque through the anti-roll bars 20 and 26 to cancel the roll of the vehicle. For example, when the spool in the valve 72 is shifted in a downward direction in FIG. 2, both of the pistons are urged in a downward axial direction. Conversely, when the valve shuttle is shifted to the left in FIG. 2, both of the pistons are urged in an upward direction.
Additional details of the active roll control system 10 shown in FIGS. 1 and 2 are included in U.S. patent application Ser. No. 11/000,319, which is incorporated herein by reference.